In general, a power supply for a video device such as a camera includes a power stage, for example, having a power transistor and a decoupling and filter capacitor connected in series on a transistor emitter collector path. This power stage is composed of discrete components due to the high value (on the order of 10 .mu.F) of the decoupling and filter capacitor, and power transistor cooling needs. A control input to the power stage, for example, the base of the power transistor, is connected to a control and regulation circuit. This circuit is preferably integrated on a chip. This circuit includes a regulation loop that receives a signal (for example a voltage) on an input, representing the value of the regulated power supply output voltage, and with an output coupled to the control input to the power stage. In a known manner, this loop reacts in order to minimize a difference between the real value of the output voltage and a theoretical voltage which is the required nominal value of the power supply output voltage.
Apart from its regulation function, this integrated control and regulation circuit also protects the power supply and powered circuits in the case of a short circuit, and limits the value of the current to a value acceptable by all elements in the circuit when the power supply is switched on. Protection against short circuits may be provided in a known manner by adding a resistor in the output circuit and by measuring the value of the voltage at the terminals of this resistor. An excessively high voltage would switch the power supply off via known circuits.
There are at least two disadvantages with this known manner of limiting the value of the current and therefore providing protection against short circuits and/or current limitations. Firstly, the voltage actually available for powered circuits is reduced by the value of the voltage drop in the resistor. This reduction varies as the value of the current output by the power supply, which causes fluctuations in the voltage actually available for circuits on the load side. Furthermore, picking up the value of the real voltage at the resistor terminals requires an additional external connection to this circuit, and it is known that the total number of connections in an integrated circuit is limited.
Another known manner of avoiding short circuits is to use a voltage comparator. The problem is to ensure that the comparison is not carried out until after the start up and stabilization phase, in order to avoid switching the power supply off immediately after it was switched on.
Furthermore, a control and regulation circuit, particularly for a camera with a port for communication with a universal standard bus (USB), includes constraints on the power supply. Thus, the current must never exceed a limiting value, for example 500 mA, particularly during start up. However, normal use may require current values of up to 300 mA. Therefore, this is a power supply in which the authorized current range is very close to the prohibited current range. The power supply must also have the normal protections against short circuits while enabling the power supply to be switched on, which (as explained above) involves two contradictory requirements.